I have just been handed a Benjamin Trail NP .177 that belongs to my father inlaw. In has been under his bed for a while I guess.

The picatinny rail has detatched from the rifle. Looks like it was attached with some kind of epoxy glue from the factory.

How best can this rail be reattached?

You might ask that question here https://www.airgunnation.com/
They could tell you what you need to reattach it

locktite blackmaxx!!!!

I see you are in Zimbabwe, so products might be difficult for you to source? If so, I suspect any of the better quality epoxy adhesives that come in two tubes and you mix together would work fine. About the only other advice is to make sure you place it exactly over the original spot ( not absolutely necessary but the manufacturer chose that spot for a reason). Since it is a picatinney rail you can remove the scope and put it back to the same place. Use tape or rubber bands to hold it in place while the epoxy sets up, a trick is to loop several rubber bands together then stretch & tie it like a piece of string.